1. Field of the Invention
The present invention relates to a linear vibration motor for generating reciprocating vibration, and more particularly to a linear vibration motor using a rechargeable battery as a power source.
2. Description of the Related Art
A conventional linear vibration motor, as disclosed in U.S. Pat. No. 6,351,089, which can be used as a drive source for a reciprocating type electric shaver is controlled in operation such that movement (displacement, velocity or acceleration) of a vibrator is detected so as to continuously generate a constant amplitude vibration even with load fluctuations, thereby generating a reciprocating movement with stable amplitude.
FIG. 9 shows a circuit diagram of a conventional linear vibration motor. As shown in FIG. 9, the conventional linear vibration motor comprises: a stator 101 provided with a winding 110; a vibrator 102 provided with a permanent magnet 120; a frame 103 for holding the vibrator 102; springs 104 connected between the frame 103 and the vibrator 102 for suspending the vibrator 102; a control output unit 105 for supplying a drive current to the winding 110; and a movement detection unit 106 for detecting the movement of the vibrator 102 on the basis of the electromotive force generated in the winding 110. Based on the output of the movement detection unit 106, the control output unit 105 controls, by e.g. PWM (pulse width modulation) control, a drive current to be supplied to the winding 110.
On the other hand, such linear vibration motor can also be used as a driving source for an electric hair cutter (hair clipper) which is operated continuously for a relatively long time. However, in the case where the linear vibration motor uses a rechargeable battery as a power supply, a voltage drop of the rechargeable battery is likely to become a problem when it is operated for a long time. In such case, it is desirable that electric power can be supplied to both the rechargeable battery and the linear vibration motor during the operation of the linear vibration motor.
In such linear vibration motor, it may possible to provide with a power supply unit for supplying electric power from an AC power source in addition to the rechargeable battery, in order to make it possible to supply electric power to both the rechargeable battery and the linear vibration motor as described above.
In this case, if the power supply unit is designed assuming a large current to flow therethrough at the time of starting the operation or with heavy loads, the power supply unit is likely to become very large in capacity and size. For this reason, even when the power supply unit is connected, it is often designed to use the power supply of the rechargeable battery as well at the time of starting the operation or with heavy loads, not electrically separating the rechargeable battery.
Now, assuming that the power supply unit supplies electric power to not only the control output unit but also the rechargeable battery during the operation of the linear vibration motor, the following three cases are considered possible with respect to the relation between the load current (average value) of the linear vibration motor and the output current (average value) of the power supply unit:                (i) [Load current of linear vibration motor]<[Output current of power supply unit]        (ii) [Load current of linear vibration motor]=[Output current of power supply unit]        (iii) [Load current of linear vibration motor]>[Output current of power supply unit]        
In case (i), the rechargeable battery is charged to increase its voltage. In case (ii), the rechargeable battery is not charged (the voltage of the rechargeable battery does not change), while in case (iii), the rechargeable battery is discharged to reduce its voltage.
If the state of case (i) continues, the rechargeable battery ultimately becomes overcharged. If the overcharge state further continues, it causes heat generation and fluid leakage of the rechargeable battery, thereby shortening its life. Accordingly, for avoiding the overcharge state, it may be required to vary the amount of electric power supplied by the power supply unit so as to satisfy the relation of case (ii) or (iii) during the operation of the liquid vibration motor.
In order to vary (reduce) the amount of supplied electric power during the operation of the linear vibration motor, it can be considered to change continuous power supply to intermittent power supply. However, such change causes the rechargeable battery voltage to increase as compared with that before the supply. Then, the current flowing through the winding increases as compared with that before the supply, and thus the magnetic thrust generated thereby increases, thereby increasing the amplitude displacement of the vibrator 102.
While, the control output unit 105 controls and stabilizes the amplitude of the vibrator 102, so that it adjusts a subsequent drive output in an attempt to reduce the increased amplitude to a predetermined value. Then, it may occur that the control output unit 105 excessively reduces the amplitude of the vibrator 102. When the increase of amplitude displacement and the excessive amplitude reduction of the vibrator 102 are repeated, it causes amplitude fluctuations, which make a whirring noise.